Automobile gas turbine engines have to cover a wide range of engine speeds from high to low, and in order to maintain acceleration performance when the engine is started, the idle running speed of the engine should preferably be high. In the Final Report (NASA CR-1808941) of the Advanced Gas Turbine (AGT) Technology Development Project, for example, the air inlet to the compressor is given a circular cross-section centered on the rotation shaft of the compressor, numerous vanes being provided on its outer circumference so as to guide inflowing air in the rotation direction of the impeller. This guided air maintains the driving horsepower of the compressor at a low level while the idle running speed is high, thereby reducing fuel costs and improving acceleration performance. The setting angle of these vanes may be modified according to the engine running conditions, the vanes being most tilted when the engine is running idle.
However, when the engine is operated with the movable vanes at a large tilt angle, noise in the frequency range 700-750 Hz is produced by the vanes. This is thought to be due to the combination of Karman vortices with disordered phases from the ends of the vanes, leading to the generation of acoustic standing waves. The noise occurs in a range above a certain air flowrate and above a certain vane tilt angle.
To combat this problem, according to the previous report, flat tabs adjacent to certain of the movable vanes were arranged to project into the air inlet. These tabs were provided at two locations separated by a 180 degree interval. The effect of the tabs is to disturb the Karman vortices produced by adjacent vanes, preventing phase addition and hence the generation of acoustic standing waves.
However, to prevent the movable vanes adjacent to the side of the tabs from interfering with the tabs, the interval between the vanes has to be increased. Further, if the direction of the induced air flow is reversed, the movable vanes adjacent to the tabs interfere with the tabs. In other words, the provision of tabs adjacent to the vanes placed severe limitations on the interval between the vanes and the range of possible vane tilt angles, and impaired freedom of design of the compressor inlet.